1. Field of the Invention
The present invention relates to pipette devices for transferring a predetermined amount of liquid samples such as of blood, plasma, serum, or urine from a liquid reservoir to a predetermined vessel or the like. More particularly, it relates to a pipette device of the type to be used in an automatic device for transferring a small amount of liquids for use in a so-called automatic analyzing device or the like in which the liquids are transferred automatically and successively so as to be subjected to a predetermined analysis.
2. Description of the Related Art
Transferring of liquids is conventionally performed by using a separate disposable nozzle tip (hereinafter: referred simply to as a nozzle tip) for each single operation in order to avoid contamination of the nozzle tip and of a sample by another. For its mounting and fitting, a pipette device of the type having a shaft body with a tapered lower end (becoming slender by an appropriate angle toward the terminal end) is used.
In such a device, the shaft body is pushed into an opening of a nozzle tip having the shape of an inverted cone, to achieve fitting based on friction due to the restoring force of an elastic deformation which occurs on the nozzle tip. Further, there is a device of the type for example disclosed in Japanese Utility Model Laid-Open No.63-175865, which uses the friction due to a restoring force of the elastic deformation of an O-ring provided at the terminal end of the shaft body.
In the above described conventional devices, however, a large force is necessary in order to push the shaft body into the nozzle tip to obtain a sufficient frictional force for the fitting thereof. As a result, a relatively large driver unit for the downward movement is required, where the pipette device as a whole is inevitably increased in size. Further, if a strict control is not provided on the relative position between the shaft body and the nozzle tip so as to achieve coincidence of the axis of the shaft with the center of the nozzle tip, the lower end portion of the shaft body is stopped by a nozzle tip inner wall and it cannot be brought into a close proximity with the nozzle tip however large the pushing force is. As a result, it is impossible to obtain an adequate fitting thereof and the nozzle tip cannot be mounted thereon--or a sufficient seal cannot be achieved be%ween the lower end portion of the shaft body and the nozzle tip where an accurate suction and ejection of liquids is impossible.
In the device which is provided with an O-ring, due to the softness of the O-ring, the degree of insertion of the shaft body into the nozzle tip varies depending on the force by which the shaft body is pushed into the nozzle tip. As a result, variance occurs in the relative position between the shaft body and the nozzle tip, i.e., distance from the lower end of the shaft body to the terminal end of the nozzle tip. It becomes difficult to control the relative position between the surface of the liquid to be taken in and the nozzle tip terminal end. Further, the coefficient of friction between the O-ring and the nozzle tip is relatively large. If, therefore, coincidence of the shaft axis and the center of the nozzle tip is not achieved, the O-ring is stopped at some point in its course and the nozzle tip cannot be mounted however large is the pushing force. There is also a problem relating to maintenance such as the periodical inspection and replacement of the O-ring.